1. Field of the Invention
The present invention relates to a valve device, and more particularly to a valve device for use as an exhaust gas recirculation valve, which has a housing accommodating an actuator and a sensor case accommodating a sensor, the housing and the sensor case being fixed integrally with each other.
2. Description of the Related Art
Exhaust gas recirculation valves have heretofore been used to remove harmful emissions discharged from internal combustion engines. The exhaust gas recirculation valve is capable of bringing the intake and exhaust systems of the internal combustion engine into communication with each other in order to recirculate exhaust gases emitted from the internal combustion engine into the intake system for thereby reduce toxic pollutants such as NOx contained in the exhaust gases.
Generally, the exhaust gas recirculation valve comprises a valve body for selectively opening and closing a recirculation path which provides communication between intake and exhaust systems of the internal combustion engine, an actuator for actuating the valve body, and a sensor for detecting an open/closed state of the valve body. The actuator comprises upper and lower stators and a coil bobbin disposed between the upper and lower stators. The actuator is accommodated in a metal housing, and the sensor is housed in a synthetic resin case.
The metal housing and the synthetic resin case are fixed and connected to each other such that the synthetic resin case is molded over the coil bobbin accommodated in the metal housing according to an in-mold forming process, as shown in FIG. 3 of U.S. Pat. No. 5,460,146. The upper and lower stators and the coil bobbin are fixed within the metal housing.
Since the synthetic resin case is molded over the coil bobbin according to an in-mold forming process, the metal housing and the synthetic resin case cannot absorb stresses that are developed at the junction therebetween due to the difference between the coefficients of thermal expansion of the metal housing and the synthetic resin case. Therefore, the metal housing or the synthetic resin case tends to crack or creep. Another problem is that it is a complex process to assemble the exhaust gas recirculation valve. Once the metal housing and the synthetic resin case have been joined to each other, it is difficult to disassemble them for reuse.
The metal housing and the synthetic resin case may be fixed and connected to each other such that flanges on joints of the metal housing and the synthetic resin case are secured to each other by crimping members having teeth that bite into the synthetic resin case, as shown in FIG. 1 of U.S. Pat. No. 5,588,414.
In order to prepare for thermal expansion of the metal housing and the synthetic resin case, the flanges thereof need to be secured by the crimping members while a large load is being applied to the crimping members. Consequently, the process of fixing and joining the metal housing and the synthetic resin case to each other is complex. Upon thermal expansion of the metal housing and the synthetic resin case, the crimping members are liable to creep and hence lower their clamping forces. Furthermore, since the crimping members are fixed to the synthetic resin case with their teeth biting into the synthetic resin case, the crimping members and the synthetic resin case will not be available for reuse after the exhaust gas recirculation valve is disassembled.